The present invention relates to a cooling apparatus, and more particularly to a cooling apparatus for cooling an optical element in an exposure apparatus that exposes an object, such as a single crystal substrate and a glass plate for a liquid crystal display (“LCD”). The present invention is suitable, for example, for an exposure apparatus that uses ultraviolet (“UV”) and extreme ultraviolet (“EUV”) light as an exposure light source.
An EUV exposure apparatus has recently been being developed as a next generation exposure apparatus that uses EUV light with a wavelength between 5 and 20 nm, e.g., wavelengths 13 nm and 11 nm, to transfer a circuit pattern with a practically minimum critical dimension (or device rule) of 50 nm or smaller onto a substrate, such as a wafer, in a lithography for manufacturing semiconductor devices.
The EUV light has a wavelength less than 10% of that of the conventional UV light, such as mercury lamps and excimer laser, and is subject to strict optical requirements. Moreover, the EUV light cannot transmit through conventional glass materials, such as quartz and calcium fluoride, due to its wavelength characteristics, and is incompatible with the conventional transmission or refractive optical system. Therefore, the EUV lithography uses a mirror even for a mask. A film formed on the mirror mainly uses a coating multilayer made of molybdenum (Mo)/Silicon (Si), but this multilayer has different reflectance, such as about 70%, according to angles of incident light. The non-reflected light among the incident light is absorbed as a heat source in a mirror, and thermally deforms the mirror surface disadvantageously. The mirror uses a material having a small coefficient of linear expansion to reduce mirror's deformations caused by temperature changes. Since the EUV exposure apparatus is used to expose a circuit pattern of 0.1 μm or smaller and required to meet very high critical dimension accuracy, only a deformation of about 0.1 nm or smaller is permissible on the mirror surface. When the mirror has a coefficient of linear expansion of 10 ppb, the mirror surface deforms as the temperature rises and the mirror surface shape changes by 0.1 nm when the temperature rises by 0.2° C. Japanese Patent Application Publication No. 11-243052 has accordingly proposed, as one solution for this problem, a method that brings a Peltier element, a heat pipe, etc. into direct contact with the mirror and cools the mirror through heat conduction.
However, the direct contact of the Peltier element, a heat pipe, etc. with the mirror, proposed in Japanese Patent Application No. 11-243052, would possibly deform the mirror surface due to vibrations of fluid, etc. that is introduced into a cooling jacket. The heat pipe applies stress onto a direct contact portion on the mirror surface, and deforms the surface. As discussed, the deformed mirror is not preferable because it deteriorates the imaging performance and does not provide a desired critical dimension.